Assessment of Reducing Pollutant Emissions in Selected Heating and Ventilation Systems in Single ‐ Family Houses

: The article presents the results of a study aiming to select the optimal source of heat for a newly designed single ‐ family home. Commercial software was used to compare heating and ventilation systems involving a bituminous coal boiler, a condensing gas boiler, a biomass boiler, a heat pump with water and glycol as heat transfer media. The effectiveness of natural ventilation, mechanical ventilation with a ground ‐ coupled heat exchanger, and solar heater panels for water heating were evaluated. The analysis was based on the annual demand for useful energy, final energy, and non ‐ renewable primary energy in view of the pollution output of the evaluated heating systems. The analysis revealed that the heat pump with water and glycol was the optimal solution. However, the performance of the heat pump in real ‐ life conditions was below its maximum theoretical efficiency. The biomass boiler contributed to the highest reduction in pollutant emissions (according to Intergovernmental Panel on Climate Change Change guidelines, carbon dioxide emissions have zero value), but it was characterized by the highest demand for final energy. Mechanical ventilation with heat recovery was required in all analyzed systems to achieve optimal results. The introduction of mechanical ventilation decreased the demand for final energy by 10% to around 40% relative to the corresponding heating systems with natural ventilation.


Introduction
According to the definition provided by the World Health Organization (WHO), air pollution occurs where the chemical composition of the air may adversely affect the health of humans, animals, and plants as well as other elements of the environment, e.g. the soil, water, or climate [1,2]. The main air pollutants include gases and particulate matter. Pollution occurs when the pollutant content of the air is exceeded in relation to the background [3].
The negative effects of air pollution on the human body have been repeatedly studied and confirmed. Research results have demonstrated increased mortality and a progressive reduction in human life expectancy [7,8]. The data indicate that approximately 3.7 million people die every year due to this reason [9]. It was demonstrated that air pollutants contribute inter alia to respiratory diseases including lung cancer and cardiovascular diseases [10][11][12][13][14]. An adverse effect of pollutants on the environment including water and soil was proven as well [15][16][17][18]. It is an important measure in this regard to conduct continuous research and to disseminate its findings, especially concerning the areas of consumer choices, whose consequences may affect the environment for decades to come [19,20].
Energy generation is one of the main sources of pollutant emissions. Households consume 12% of globally generated energy [21] and indoor heating accounts for a significant portion of energy consumption [22]. A review of the research conducted in recent decades and the proposed solutions reveals three main trends [23]. The first involves attempts to decrease the operating time of electrical devices by changing user habits. The second is the improvement in the thermal insulation of buildings, and the third involves the replacement of outdated equipment with new and energyefficient devices [24]. Two trends predominated in recent years. First, attempts have been made to shift energy consumption to off-peak hours when the emissions from energy generation are the lowest [25,26]. Second, low-emission heating systems that rely on renewable energy sources have been popularized in newly developed or upgraded buildings [27,28].
Energy consumption patterns in households are influenced by climatic, demographic, economic, and lifestyle factors [29]. In European countries with a cold climate, including Poland, other than the Baltic States and Nordic countries, climatic factors play a key role because they prolong the heating season. Economic factors are also a very important consideration in Poland and other former Soviet block countries where income levels are lower, which explains the widespread use of conventional heating systems based on non-renewable fossil fuels [30]. These factors have been taken into consideration in this study. Efforts have also been made recently to lower household emissions by installing modern heating systems that fully meet the users' needs, decrease the heating carbon footprint, and are affordable.
Research studies addressing these topics are highly dispersed, and they have to account for regional factors, such as the availability of different energy sources and the relevant costs. In general, the energy market is highly diverse. In Finland, 48% of households have electric heating. Central heating systems are not popular, and most households are equipped with electric heaters [23]. In contrast, the heating systems in 80% of German households are fired with heating oil and gas [31]. The Polish heating market differs from other European countries in that it still relies heavily on coal. Therefore, research studies should account for local and regional factors, in particular in large countries such as Poland, to formulate the most effective recommendations for regions with similar climatic conditions and similar levels of economic growth. In Poland, the existing research is also dispersed, and most studies compare the emissions from modern heating systems that rely on renewable energy sources with conventional systems that are fired by fossil fuels. However, the number of the analyzed variants and/or pollutants are relatively small, which narrows down the scope of these analyses [30,[32][33][34]. Some studies focus on a single energy source or a single pollutant [35,36]. Research into residential heating systems often fails to account for the impact of ventilation on pollutant emissions. For this reason, the study aimed to present and analyze the most popular combinations of heating and ventilation systems in view of the generated emissions and the associated costs.

Attempts to Solve the Pollutant Emission Problem
The European Environment Agency (EEA) data [37] indicate that the most polluted air in Europe is found in urban areas where 73% of the population lives. This value may increase to 82% in 2050 [38]. The problem is both important and complex, particularly in the context of the possibility of free spreading of the air and its pollutants over long distances from their emission sources.
The increasing awareness of hazards results in the development of environmental protection policy, and the establishment of relevant supervisory institutions [39,40]. The awareness of responsibility for future generations is increasing as well [41][42][43][44].
For many years, European Union institutions have made numerous efforts to improve air quality, with the aim of achieving a level that will have no adverse effects on human health or the environment [13,45,]. One of the major initiatives is the "Clean Air" program which aims to raise the existing standards for emissions and air pollutant limits through legislative action. This strategy foresees comprehensive and consistent actions and measures to reduce pollutant emissions [46]. In 2015, EU Member States adopted Directives which set national emission limits for particulate matter (PM10 and PM2.5) and gaseous pollutants (sulfur and nitrogen oxides, ammonia, and non-methane volatile organic compounds).
The need for improvement in the air pollution situation is also responded to by the WHO which produced recommendations concerning the maximum daily values for the PM10 and PM2.5 particles that should not be exceeded more often than three times a year within a particular area [47].
Coordinated actions are slowly yielding results. According to the Eurostat studies [48], in the years 2006-2015 the average concentrations of most harmful substances in the air decreased. Despite the improvements, the situation is not resolved completely, and in many areas, the polluted air continues to harm human health and the environment. The European Union comprises 28 countries that are very diverse in terms of the economy, law, and climate. The needs and opportunities with regards to the policy concerning, e.g. energy production, which, according to study results, has a significant impact on pollutant emissions in particular countries, are also very different [49][50][51]. Therefore, the air pollution levels and the pollutant structure differ significantly [52,53]. As research shows, for example, countries with a small area and/or low population density may emit the same amounts of pollutants to the atmosphere as large densely populated ones [54]. In general terms, the greatest amounts of gaseous pollutants are emitted to the atmosphere by Germany, France, Italy, and Spain, while most fine particulate matter is emitted by France, Italy, and Poland [54]. In Poland, particulate matter concentration levels in the air can be eight (for PM10) and six (for PM2.5) times higher than those indicated by the WHO guidelines [55].  [59].
Based on the regulations and guidelines, Poland's report presenting the balance of air pollutant emissions reportable to the UN/ECE Convention for the years 2015-2017 was once again drawn up and published in 2019 [60].
The report indicates an upward trend for emissions of most of the main pollutants except sulfur dioxide. A list of emission levels and a comparison for the years 2015-2017 are presented in Table 1. The table shows that compared to 2016, the emissions that increased the most in 2017 were nitrogen oxide (by 8.3%) and particulate matter (black carbon-BC by 12.2%). Sulfur dioxide emission decreased by 1.4%, while compared to 2015, by 18.1%. Emissions of most persistent organic pollutants also decreased, particularly those of hexachlorobenzene HCB (by approximately 2%); on the other hand, polychlorinated biphenyls (BCB) emissions increased slightly (by approximately 0.05%).
It should be stressed that for many pollutants (in particular nitrogen oxides, non-methane volatile organic compounds NMVOC, ammonia NH3, carbon monoxide CO, black carbon BC, copper, and particulate matter), significant increases in emissions occurred over a two-year period [61].
Further analysis of the balance in 2017 indicates the problem associated with non-industrial combustion processes where the largest proportion of emissions results from the combustion in households. It was demonstrated that on a national scale, they emit 29.3% sulfur dioxide (SO2), 10.7% NOx and 22.3% black carbon (BC). The situation is even less advantageous regarding emissions of PM2.5 particulate matter (46.5%), carbon monoxide CO (59.2%), total suspended particulate matter TSP (44.6%), PM10 particulate matter (46.5%), and polychlorinated biphenyls PCB (68.4%) or polycyclic aromatic hydrocarbons PAH (83.7%) [58].
As research shows, electricity generation, particularly from fossil fuels, is most responsible for pollutant emissions [50,61]. Increasing research and efforts are focused on the development of a lowcarbon economy. In addition to energy savings, one of the proposed solutions is to increase the use of energy from renewable sources. One study demonstrated that a net increase in energy production based on renewable sources by 1% results in a reduction in carbon dioxide, one of the most dangerous greenhouse gases, by 0.16% [62].
In Poland, one of the strategic objectives is a reduction in greenhouse gas emissions, supported by the use of renewable energy sources as well as pro-efficiency measures in the energy sector. In the years 2006-2015, while the energy sources based on primary energy gradually decreased, the use of renewable sources increased. In 2015, the proportion of energy from renewable sources amounted to 11.8% of total energy consumption, which ranked it 21st among EU countries.
However, hard coal and brown coal were still the major sources of energy in 2018, and their share of electricity generation reached 47.8% and 29.0%, respectively, marking a decrease of 9.8% from 2010. Renewable energy sources accounted for 12.7% of energy production, and their proportions in the power mix increased by 5.8% from 2010. The main sources of renewable energy were wind energy, biomass, and biogas. Solar energy had the smallest share of energy generation, but it was characterized by the highest growth dynamics [63]. However, a steady upward trend towards the use of renewable sources should be emphasized [64].
The article describes a study whose results will enable the selection of the optimal (also in terms of pollutant emissions) source of heat for a newly-designed single-family residential building. The analysis was developed based on research carried out in the city of Olsztyn, the capital of Warmińsko-Mazurskie Voivodeship.
Warmińsko-Mazurskie Voivodeship is the fourth largest voivodeship in Poland. Its area of 24,173 km 2 is inhabited by 1429 people (as of 31st December 2018) [65].
Compared to the rest of the country, a few industrial plants that are particularly harmful to the environment are located in this voivodeship. Emissions from point sources are mainly concentrated in localities which are seats of poviats (administrative units). Due to the poorly-developed transport network, a decrease in linear emissions is noticeable in localities and poviats in which several roads with high traffic volume intersect, and in the largest cities i.e., Elbląg and Olsztyn. The greatest pressure is exerted by surface emissions, i.e., those from individual heating systems. For example, almost 67% of PM10 particulate matter emitted within the voivodeship originate from this source, while approximately 8% each originate from farming, point sources, and road emissions. The relatively small proportion of surface emissions is within the city of Olsztyn and amounts to 18%. It is worth noting that the city has been implementing its own programs to combat air pollution. A recent initiative is the "Replace the Stove" project, which is a part of the "Clean Air Priority Programme." Its aim is to reduce or prevent emissions of particulate matter and other pollutants introduced into the atmosphere by single-family houses. Municipal authorities co-finance the replacement of heating sources in residential buildings with environmentally friendly sources. What is important is the program is addressed to both natural persons who own single-family houses and to persons holding a permit for the commencement of construction work. Financial support is also available to tenants' associations interested in obtaining grants for connecting properties to the district heating system [66].
The selection of the optimal heat/energy source in this particular region is especially justified due to the climatic conditions. Warmińsko-Mazurskie Voivodeship is located in a lowland area that is the coldest in the whole country. It is characterized by a high forestation rate and lake density. The climate is described as transitional, maritime/continental climate with average annual temperatures ranging from 6 to 8 °C [67,68]. This is the greatest area and one of the few in Poland where the growing season with the average daily temperature above 5 °C is the shortest. With the national average for the years 1971-2000 which amounts to 218 days of the growing season, in the Warmia and Mazury region it lasts less than 200 days, and in certain locations, it is shorter than 190 days [69].
Such climatic conditions result in a significant, compared to other regions, an extension of the heating system which, in turn, extends the period of increased pollutant emissions due to the increased demand for thermal energy. Given the relatively low levels of the region's industrialization and transport, the production of energy, including thermal energy in individual households is the major source of harmful substance emissions to the air.
The newly designed single-family building in areas similar, in climate terms, to the region of north-eastern Poland, must ensure unlimited access to domestic hot water (DHW) and fully satisfy the increased energy demand for thermal comfort.

Materials and Methods
The study aimed to identify the optimal combination of a heating system and a ventilation system characterized by low emissions for newly developed or upgraded single-family homes. Combinations involving heat sources that are most popular in the Polish market were analyzed. The energy efficiency of the analyzed systems was compared, and the results of theoretical analyses were confronted with real-world parameters to ensure that the selected solution is best adapted to local conditions.
A newly designed single-family building located in Olsztyn was analyzed. The building is a realworld object which is scheduled for construction in 2020. The floor space of the building is 152.76 m 2 . The building is designed to be inhabited by four people. For the construction, materials were used that enable obtaining the maximum heat penetration coefficient values for space dividing elements in accordance with the Regulation of the Minister of Infrastructure and Construction of 2017 [70]; these amount to, respectively:  for the ground slab: 0.30 (W/(m 2 K))  for external walls: 0.20 (W/(m 2 K))  for the flat roof: 0.15 (W/(m 2 K))  for external windows: 0.9 (W/(m 2 K))  for external doors: In the initial variant, gravity ventilation and a conventional hard coal boiler with 82% efficiency were used for the purposes of central heating and the preparation of domestic hot water (DHW).
ArCADia-TERMOCAD software by INTERsoft (ArCADiasoft, Łódź, Poland) [71] was used to draw up an energy performance certificate for the building, and the environmental effect for the heating and ventilation system variants presented in Table 2 was compared. Additionally, for the purposes of domestic hot water generation, each of the variants considered the possibility of extending the installation system to include a system of flat-plate and vacuum collectors.
The analyzed heating and ventilation variants for the single-family home with an individual heating system were based on the most popular solutions in Poland, including a hard coal boiler, a condensing gas boiler, and a biomass boiler.
The heating and ventilation variant involving a heat pump was selected because the number of heat pump systems installed in Poland increased by 20% in 2018. At present, every seventh new building is heated by a heat pump [72]. Solar thermal collectors for water heating were included in the study due to the success of the "Support for dispersed renewable energy sources" program of the National Fund for Environmental Protection and Water Management [73]. Solar collectors with a combined area of 475,207 m 2 have been installed as part of the program [74]. In 2018, 1.29% of Polish households were equipped with solar collectors and 0.48% with heat pumps [75].
Single-family homes with natural ventilation are characterized by very high energy losses, and this fact was taken into consideration in the analyzed scenarios.
Based on the analysis of the determined value of the annual demand for usable energy (UE), final energy (FE), and non-renewable primary energy (PE), and the pollutant emission indices for particular systems (SO2, NOX, CO, CO2, particulate matter, black carbon, and B-a-P), the optimal variant of the heating and ventilation system was indicated.
The values of usable energy (UE), final energy (FE), and primary energy (PE) were determined based on the methodology described by the Regulation of the Minister of Infrastructure and Development of 27 February 2015 on the methodology for calculating the thermal characteristics of a building or a part of a building and energy attribute certificates [76]. In heating systems, usable energy (UE) is defined as the energy that is evacuated from a building via the ventilation system, minus heat gain. In water heating systems, UE is defined as the energy evacuated from a building via sewage. Usable energy is calculated with the use of the below formulas: where: -annual demand for usable energy, (kWh/year) -floor area of premises with controlled air temperature, (m 2 ) , -annual demand for usable energy for heating and ventilation, (kWh/year) , -annual demand for usable energy for water heating, (kWh/year) , -annual demand for usable energy for cooling, (kWh/year) Final energy (FE) is defined as the energy supplied to technical systems in a building, and it is calculated with the use of the below formulas: where: -annual demand for final energy supplied to technical systems in a building, (kWh/year) , -annual demand for final energy supplied to a building or a part of a building for heating purposes, (kWh/year) , -annual demand for final energy supplied to a building for water heating, (kWh/year) , -annual demand for final energy supplied to a building for cooling purposes, (kWh/year) , -annual demand for final energy supplied to a building for lighting common areas; this parameter is not calculated for single-family homes and apartments, (kWh/year) , -annual demand for auxiliary energy supplied to technical systems in a building, (kWh/year) Non-renewable primary energy (EP) is defined as the energy which is stored in fossil fuels and has not been converted or transformed: where: -annual demand for non-renewable primary energy for technical systems, (kWh/year) , -annual demand for non-renewable primary energy for heating systems, (kWh/year) , -annual demand for non-renewable primary energy for water heating, (kWh/year) , -annual demand for non-renewable primary energy for cooling systems; (kWh/year) , -annual demand for non-renewable primary energy for lighting common areas; this parameter is not calculated for single-family homes and apartments, (kWh/year) Specific CO2 emissions are calculated with the use of the below formulas: where: , -CO2 emissions from fuel combustion in heating systems, (tCO2/year) , -CO2 emissions from fuel combustion in water heating systems, (tCO2/year) , -CO2 emissions from fuel combustion in cooling systems, (tCO2/year) , -CO2 emissions from fuel combustion for lighting installations, (tCO2/year) where each of the above components (n) is calculated with the use of the below Equation (8): , -annual demand for final energy supplied to the analyzed system in a building, (kWh/year) , -CO2 emissions from the combustion of different types of fuel in the analyzed systems, (tCO2/TJ) , -CO2 emissions from fuel combustion in heating systems (tCO2/year) is calculated with the use of formula (10): , , -annual demand for auxiliary energy supplied to the n th system in a building, where H is the heating system, W is the water heating system, and C is the cooling system, (kWh/year) , , -CO2 emissions from the combustion of different types of fuels in the n th system, where H is the heating system, W is the water heating system, and C is the cooling system, (tCO2/TJ) The annual consumption of energy carriers or electricity, district heating, solar energy, geothermal energy, wind energy, and gas is calculated with the use of formula (10): The consumption of different energy carriers and energy types than those indicated in formula (10) is calculated with the use of formula (11): where: -heating value of fuel, (MJ/m 3 or MJ/kg) Pollutant emissions (Table 3) are calculated based on the reference materials published by the Ministry of Environmental Protection, Natural Resources and Forestry [77] regarding emissions of air pollutants from fuel combustion, as well as the guidelines of the National Centre for Emissions Balancing and Management (KOBiZE) [78] with the use of the below formula: • where: -fuel consumption, (m 3 or Mg) -pollutant emissions per unit of consumed fuel, (kg/Mg or kg/10-6 m 3 or kg/kWh) The efficiency and effectiveness of the devices installed at the University of Warmia and Mazury Research Laboratory and operating under actual weather conditions were taken into account. The feasibility of the assumptions of theoretical calculations was compared with the possibility for the use of a heat pump, mechanical ventilation with ground-coupled heat exchanger (GCHE), and flatplate and vacuum collectors in the heating and ventilation systems.

Results and Discussion
The annual demand for usable energy for heating and ventilation in the analyzed building is 8075 kWh/year for the design variants with gravity ventilation (1, 3, and 5) and 1997 kWh/year for mechanical ventilation (variants 2, 4, and 6). Gravity ventilation enforces the extension of the period of heating system used in the building to 4675.88 h, compared to 3002.51h for mechanical ventilation systems. Heat losses for ventilation amount to slightly more than half the heat losses in the balance for the entire building for cases with natural ventilation, and approximately 6% where mechanical ventilation is used ( Table 4). The comparison also shows the seasonal variability of the heating and ventilation system operation.  Analysis of the EU values obtained in the study for particular cases (Figure 1) showed nearly twice its value for the variants with gravity ventilation compared to solutions with mechanical ventilation with GCHE. This results from low heat losses due to the advantageous heat penetration coefficient values for space dividing elements used in the building. Therefore, the proportion of losses related to the natural ventilation of the building is noticeable in the overall balance.
Systems with the lowest efficiency are characterized by the highest FE. In terms of this parameter, the most advantageous systems include heat pump systems (overall efficiency of 2.66), systems with a low-temperature natural gas condensation boiler (overall efficiency of 0.76), coal boiler systems (overall efficiency of 0.60), and biomass (wood) boiler systems (overall efficiency of 0.51). FE for the analyzed energy sources is lower by approximately 40% than for mechanical ventilation. Heat pump systems (case 7 and 8) for which the decrease is approximately 7% are an exception.
In terms of the demand for FE and the exploitation of non-renewable sources, biomass-using systems are the most advantageous. In this single case, the use of mechanical ventilation with the unchanged heat source in the building results in FE being higher than for the use of gravitational energy. In other systems, the introduction of mechanical ventilation results in a reduction in FE by approximately 10% for cases 7 and 8, by approximately 18% for cases 3 and 4, and by approximately 23% for cases 1 and 2. In terms of PE, systems with a coal boiler source are the most energy-intensive. In case 1, the demand for PE is higher by 76% than case 5 (the most advantageous one in this variant). High energy losses related to gravity ventilation encourage the use of mechanical ventilation with GCHE in the building, which enables the maintenance of energy-efficient construction standards and the heat recovery efficiency in the air handling unit at a level above 85%. The issue of the efficiency of heat extraction from the ground remains an open question. Testing on the groundcoupled pipe heat exchanger located at the Research Laboratory, conducted under the actual operational conditions, demonstrated that continuous operation under summer and transitional conditions is ineffective [79]. On the other hand, under winter conditions, the device enables efficient heat extraction from the ground [80]. Figure 2 shows the variability of temperatures at the input to and output from the GCHE in an exemplary month (October 2018). The device operated in a continuous mode without regeneration, and the average flowing airstream was approximately 155 m 3 /h. During the analyzed period, 204.19 kWh of heat was extracted from the ground. During the period from September to March, average GCHE energy efficiency at a level of 35% was obtained, with experimentally-noted maximum hourly values of GCHE energy efficiency reaching approximately 90%.   Compared to the results for actual operation, the use of the solar collector system in the DHW (domestic hot water) system should be considered (flat-plate and vacuum solar collectors connected to a 1000 L hot water storage tank were analyzed separately). The energy required for DHW purposes is constant throughout the year. In the analyzed variants with gravity ventilation, final energy for the DHW purposes amounts to approximately 38% (cases 1, 3, 5, and 7). On the other hand, for the variants with mechanical ventilation (cases 2, 4, 6, and 8) for domestic hot water preparation, it amounts to 40%-60% in the overall final energy balance for the building. Analysis of actual 2016 measurement data for flat-plate and vacuum collectors operating at the Research Laboratory at the University of Warmia and Mazury demonstrated that the average hourly efficiency of flat-plate collectors amounted to 22% and of vacuum collectors to 37% [81]. According to the Keymark test dependence for flat-plate collectors, an efficiency of 63% was obtained, while for vacuum collectors, the efficiency of 72% was obtained. The above results differ from the catalog efficiency of 74.3% for flat-plate collectors and of 78.9% for vacuum collectors, declared by the manufacturer. During the period of the most efficient operation of the above-mentioned devices (from May to September), unit heat from flat-plate collectors amounted to 77 kWh/m 2 , while 188 kWh/m 2 was obtained from vacuum collectors. During the period from September to March, the collectors, irrespective of the type, operated sporadically, and they cannot be considered to be energy sources for the DHW [82].
The annual demand for usable energy (EU) was identical in water heating systems with and without solar collectors. The annual demand for final energy (FE) was higher in solar collectors which are less efficient than water heating systems based on a heat pump or a condensing gas boiler. In cases 7 and 8, where the use of solar collectors led to the greatest decrease in efficiency, relative to the highly efficient heat pump system, FE increased by 83% for flat-plate collectors and by 76% for vacuum collectors. In systems involving a condensing gas boiler (cases 3 and 4), FE increased by 4% when flat-plate collectors were added and by 2% when vacuum collectors were added. In the remaining cases, the introduction of flat-plate collectors decreased FE by 3%, and the installation of vacuum collectors decreased FE by 5%. If solar collectors were to supply 30% of the annual energy for water heating, the demand for non-renewable primary energy (PE) would decrease by 28% in cases 1 and 2, by 27% in cases 3 and 4, by 26% in cases 7 and 8, and by around 20% in cases 5 and 6.
During the winter period (from November to March), measurement data for the operation of a water-glycol compressor heat pump were also obtained, with the lower source in the form of vertical boreholes. The test results are presented in Figure 3. The result analysis demonstrated that the average coefficient of performance (COP) value was 3.36 and ranged from 2.24 to 4.46, while the COP catalog value declared by the manufacturer was 4.57. The emission analysis (Table 5) was conducted based on the indices of pollutant emission from fuel combustion in boilers with a nominal thermal power of up to 5 MW, according to the National Centre for Emissions Management (KOBiZE) [79]. The analysis results are presented in Table 4. The obtained results indicate that the use of a biomass (wood) boiler proved to be the least advantageous for the analyzed building due to carbon dioxide emissions. In this context, this solution appeared to be more emission-intensive than hard coal-fired boilers. It should be stressed, however, that in accordance with the IPCC guidelines [84], it is assumed in the emission trading scheme that the biomass combustion balance in this aspect has a zero value. It is assumed that the emission during combustion is compensated for CO2 uptake in the photosynthesis process. In terms of CO2 emissions, the most advantageous of the analyzed cases is the use of a low-temperature, natural gas condensation boiler, particularly in combination with mechanical ventilation with GCHE. A similar situation occurs for carbon monoxide emissions in the analyzed cases. Regarding the emissions of SO2, black carbon, and benzo(a)pyrene, the systems based on natural gas or biomass combustion proved to be the most advantageous. A condition for achieving this effect is the least possible electricity consumption, even at the expense of excluding gravity ventilation. Regarding SO2 emissions, the use of electricity feeding the heat pump (and auxiliary equipment), has consequences in emissions similar to those for a hard coal boiler. On the other hand, for the emissions of black carbon and benzo(a)pyrene, the use of hard coal is hundreds (B-a-P) or even thousands of times (black carbon) more disadvantageous.
Regarding NOX emissions, the most disadvantageous energy source is electricity (heat pump), followed by the following sources, in order: hard coal boiler, biomass boiler, and natural gas boiler. It can also be noted that an increased proportion of NOX emissions was found in the variants using mechanical ventilation requiring electricity supply (cases 2, 4 and 6), which confirms the significant effects of electricity use for the emissions of this pollutant.
Particulate matter emissions from hard coal combustion for the concerned building generates emissions higher by approximately 15% than a biomass boiler. This value is two (mechanical ventilation system) or three times (gravity ventilation source) higher than for the electricity using a heat pump. In terms of particulate matter emissions, the use of a natural gas boiler is the most advantageous.
In Table 6, the analyzed systems were classified based on pollutant emission levels, from the least to the most polluting cases. Condensing gas boilers and biomass boilers ranked highest in the presented classification. The demand for grid electricity in the energy balance increased pollutant emissions, which was most apparent in mechanical ventilation systems with heat recovery in case 4 (condensing gas boiler and mechanical ventilation) and case 6 (biomass boiler with mechanical ventilation). Grid electricity (for example, for the operation of heat pumps in cases 7 and 8) does not generate pollution in the heated building, but it involves considerable loads and losses during energy production and transfer. However, individual users can reduce pollutant emissions by installing photovoltaic panels in the household.

Conclusions
Selected combinations of heating and ventilation systems were compared and analyzed in this study. Pollutant emissions from the presented solutions were determined. Unlike most research papers addressing the discussed topic, this study not only classified selected variants based on their emissions, but it also analyzed their energy efficiency. Special emphasis was placed on mechanical ventilation with heat recovery which can reduce emission levels. In the authors' opinion, mechanical ventilation with heat recovery is often unnecessarily disregarded or marginalized. This study proposes universal solutions that can be applied to heating and ventilation systems in countries with a similar climate.
The results of this study constitute highly valuable inputs for the Polish market because the analysed variants combine heating and ventilation systems that are most popular in single-family homes in Poland. The study also evaluated the effectiveness of solar heater panels for water heating which are increasingly popular among Polish homeowners. The theoretical efficiency of heating systems powered by renewable energy sources (solar heater panels, heat pump, ground heat exchanger) was compared with their performance in real-world conditions in north-eastern Poland.
The choice of an optimal heating and ventilation system for a single-family home is a complex problem. The real-world performance of system components is generally below their maximum theoretical efficiency. Even the most advanced systems based on renewable energy sources, such as heat pump circuits filled with a water-glycol mixture, have a theoretical COP of 4.57. In practice, the maximum COP was determined at 4.46, and the average COP during the heating season reached 3.36. Geothermal energy can be effectively used to preheat cold incoming air in the ground heat exchanger. This solution is far more effective for preheating cold air in winter than for cooling hot air in summer. However, even when hourly efficiency approximated 90% in winter, the average efficiency between September and March reached around 35%. In north-eastern Poland, the efficiency of solar thermal collectors in the optimal period of operation (May to September) was 11.3% below the theoretical value in flat-panel collectors and 6.9% below the theoretical value in vacuum collectors.
Energy consumption is an important consideration when selecting a heating and ventilation system. Usable energy (UE) values are influenced by structural partitions, the presence of thermal bridges and the airtightness of a building. The ventilation system also exerts a considerable influence on EU values. In the study, a highly efficient ventilation system with heat recovery (95%) decreased energy loss from the building by nearly 50%.
The consumption of usable energy in a building can be reduced not only through the conscious efforts of architects and developers, but also through legislative measures which introduce thermal insulation standards for structural partitions as well as airtightness standards. However, the existing legal regulations do not account for the impact and consequences of heat recovery in ventilation systems. In Poland, the minimum efficiency of heat recovery devices is set at 50% for mechanical ventilation systems and air-conditioning systems with an air exchange rate of 500 m 3 /h and higher [86]. These solutions are characterized by low thermal efficiency, and they are mandatory in large buildings. Consequently, according to Statistics Poland data, only 0.26% of Polish households were equipped with mechanical ventilation systems with heat recovery in 2018 GUS (Statistics Poland) [87]. The measures aiming to improve the energy efficiency of single-family homes should not only increase the thermal performance of structural partitions (for example, through additional thermal insulation) but should also involve upgrades of heating systems. The improvement in the efficiency of central heating and water heating systems should be accompanied by the installation of mechanical ventilation systems with heat recovery.
Final energy (FE) is closely linked with the applied technical solutions and the efficiency of heating and ventilation systems. In the analyzed scenarios, systems based on heat pumps were characterized by the highest efficiency and the lowest FE values. In buildings equipped with heat pumps, mechanical ventilation with heat recovery can additionally decrease the demand for FE. This effect is most visible in buildings with low-efficiency heating and ventilation systems. Total efficiency was lowest in the system involving a biomass boiler, where the introduction of a ventilation system with heat recovery decreased the demand for FE by 41%. Final energy consumption is the key determinant of energy costs; therefore, this parameter should play the most important role for developers. However, primary energy (PE) is a vital parameter from the point of view of environmental protection and sustainable development because it denotes the amount of energy generated from fossil fuels, including mining operations and fuel transport. In this respect, case 5where heat was supplied by a biomass boiler and additional energy was not required for mechanical ventilation-emerged as the optimal variant.
The study revealed that a condensing gas boiler combined with natural ventilation was characterized by the lowest pollutant emissions, excluding carbon dioxide. Carbon dioxide emissions were lower only in the system featuring a gas boiler and mechanical ventilation. A biomass boiler with mechanical ventilation was also a relatively low-emission solution.
The life cycle balance of the proposed solutions will be analyzed in future research.